From: Junio C Hamano <gitster@pobox.com>
Date: Mon, 26 Feb 2024 23:46:44 GMT
Subject: Re: [PATCH v1 1/4] builtin/index-pack.c: change xwrite to write_in_full to allow large sizes.
Message-ID: <xmqqa5nmkcuz.fsf@gitster.g>
In-Reply-To: <026b01da6906$4d96f530$e8c4df90$@nexbridge.com>

<rsbecker@nexbridge.com> writes:

>>The code above loops while input_len is non-zero, and correctly
>>decrements it by the number of bytes written by xwrite() after
>>each iteration.
>>
>>Assuming that xwrite()/write(2) works how I think it does on
>>NonStop, I'm not sure I understand why this change is necessary.
>
> NonStop has a limited SSIZE_MAX. xwrite only handles that much so
> anything beyond that gets dropped (not in the above code but in
> other builtins)

xwrite() caps a single write attempt to MAX_IO_SIZE and can return a
short-write, so anything beyound MAX_IO_SIZE will not even be sent
to the underlying write(2).  There is a heuristic based on the value
of SSIZE_MAX to define MAX_IO_SIZE in <git-compat-util.h>, and if
the value given by that heuristics is too large for your platform,
you can tweak your own MAX_IO_SIZE (see the comments in that header
file).

The caller of xwrite() must be prepared to see a write return with
value less than the length it used to call the function, either
because of this MAX_IO_SIZE cut-off, or because of the underlying
write(2) returning after a short write.  As long as the caller is
prepared, like Taylor pointed out, I am not sure why you'd need to
change it.